With the development of communications technologies, because a PON (Passive Optical Network) system has advantages of wide bandwidth, good scalability, saving feeder fibers, omitting maintenance and power consumption of active components, and wide coverage, the PON system is increasingly widely applied in the broadband access field.
In order to split signals, which are in coming and going directions and over a same optical fiber, of multiple users, the PON system uses the following two multiplexing technologies to transmit a digital signal, which are specifically as follows:
in a downlink transmission direction (that is, from an optical line terminal to an optical network unit): the OLT (optical line terminal) sends a digital signal in a continuous broadcasting manner, and can send a digital signal to the ONU (Optical Network Unit, optical network unit)/an ONT (Optical Network Terminal, optical network terminal) at any time, all ONUs can receive same digital signals, but each ONU receives its own digital signal by means of filtering; and
in an uplink transmission direction (that is, from an optical network unit to an optical line terminal): an optical fiber is occupied separately in different time periods according to certain time length; in each time period, only one ONU can occupy the optical fiber to send a digital signal to the OLT, and the other ONUs turn off lasers, that is, the OLT can receive a digital signal sent by only one ONU at a same time period; if two ONUs simultaneously send digital signals to the OLT, the OLT cannot correctly receive the digital signals, and a conflict occurs between uplink digital signals sent by different ONUS.
With the development of communications technologies, a high transmission rate is of great importance, and system upgrade increases a transmission rate; however, an increase in the transmission rate indicates occupation of higher bandwidth, and in a case of higher bandwidth, optoelectronic components have relatively high costs. Currently, in order to reduce high implementation costs that are brought by high bandwidth after the transmission rate is increased, bandwidth generally needs to be compressed.
In an existing high rate upgrade solution, an up-conversion processing technology needs to be used in a process of performing bandwidth compression on an optical fiber channel in a PON system, and specific processing is: an OLT converts a received digital signal into a digital in-phase signal and a digital quadrature signal, processes the digital in-phase signal and the digital quadrature signal into one digital signal by using an I/Q modulator, loads the digital signal obtained after processing to an optical signal, and then, performs up-conversion processing on the optical signal, and sends the optical signal to an ONU.
However, after a digital signal is processed by using the up-conversion technology, a rate of the digital signal is very high, leading to relatively severe distortion of the digital signal, and relatively poor quality of the digital signal transmitted to an ONU side; in addition, an up-conversion processing device has relatively high costs. Therefore, during signal transmission, the prior art has a problem of relatively poor digital signal quality and relatively high implementation costs.